This invention relates to a process for producing a cobalt-containing magnetic iron oxide powder which is useful as a magnetic material for magnetic recording media.
Generally, a cobalt-containing magnetic iron oxide powder has high coercive force and magnetic tapes produced therefrom are most suited for high density recording and also have excellent sensibility in the high frequency range. For these reasons, such cobalt-containing magnetic iron oxide powder is popularly used in the field of magnetic recording media such as acoustic signals, visible images and other types of information. Recently, however, even higher densification of the magnetic recording media is much in request and, accordingly, further improvement in performance of said cobalt-containing magnetic iron oxide powder has been desired.
The magnetic iron oxides used as core particles to be coated with a cobalt-containing compound have been generally produced by a wet synthetic process, and in such production process, it has been usual to add a phosphorus compound at the time of formation of acicular hydrous iron oxide or before the heat treatment (dehydration or reduction) to prevent the deformation of particle shape or sintering and coarsening of the particles due to the heat treatment, thereby providing the product with high acicularity.
In the course of studies for further improvement of performance of said cobalt-containing magnetic iron oxides, the present inventors have found that (1) the improvement of the magnetic characteristics by coating with a cobalt-containing compound depends greatly on the properties of the core material (particles) to be coated with said compound and (2) especially in case of using a phosphorus-containing magnetic iron oxide as core material, it is hard to bring out the desired magnetic characteristics such as high coercivity to a satisfactory degree by said coating or such magnetic characteristics, if brought out, would vary widely from one product to another, and further the magnetic recording medium produced from such material tends to prove poor in thermal stability. Further studies on these points in particular have led the present inventors to the finding that all of the said problems can be solved when using as core material a phosphorus-containing magnetic iron oxide treated in an aqueous alkaline medium under pressure and coating the thus treated core material with a cobalt-containing compound, and this invention was completed on the basis of such finding.